Mechanical refrigeration systems, and related heat transfer devices such as heat pumps and air conditioners, using refrigerant liquids are well known in the art for industrial, commercial and domestic uses. Chlorofluorocarbons (CFCs) were developed in the 1930s as refrigerants for such systems. However, since the 1980s the effect of CFCs on the stratospheric ozone layer has become the focus of much attention. In 1987 a number of government signed the Montreal Protocol to protect the global environment setting forth a timetable for phasing out the CFC products. Subsequent amendments to this protocol accelerated the phase-out of these CFCs and also scheduled the phase-out of HCFCs. Thus, there is a requirement for a non-flammable, non-toxic alternative to replace these CFCs and HCFCs. In response to such demand industry has developed a number of hydrofluorocarbons (HFCs), which have a zero ozone depletion potential.
Hydrofluorcarbons such as difluoromethane (HFC-32), 1,1,1-trifluoroethane (HFC-143a) and 1,1-difluoroethane (HFC-152a) have essentially no ozone depletion potential (ODP) and low global warming potential (GWP), and therefore, they have been found to be acceptable refrigerants and, in some cases, as potential blowing agents in the production of plastic foams.
There are already known methods in the literature to produce HFC-32. U.S. Pat. Nos. 2,749,374 and 2,749,375 disclose a process wherein dichloromethane (HCC-30) is reacted with HF in a liquid phase at a temperature within the range from 110° to 175° C. in the presence of an fluorine containing antimony halide catalyst to obtain HFC-32. In this process, however, a large amount of R-40 series compounds such as monochloromethane and fluoromethane, which are undesired impurities other than R-30 series compounds (HFC-32, HCFC-31 and HCC-30), are formed as by-products. It is also well known in the art that liquefied HF and antimony halide mixtures are very corrosive to metals and alloys.
U.S. Pat. No. 2,745,886 discloses a vapor phase process for fluorinating a variety of halohydrocarbons including HCC-30, which utilizes a hydrated chromium fluoride catalyst activated with oxygen. Similarly, U.S. Pat. No. 2,744,148 discloses a halohydrocarbon fluorination process in which an HF-activated alumina catalyst is used. U.S. Pat. No. 3,862,995 discloses the vapor phase production of HFC-32 by reacting methylene chloride and HF in the presence of a vanadium derivative catalyst supported on carbon. U.S. Pat. No. 4,147,733 discloses a vapor phase reaction for the production of HFC-32 by HCC-30 with HF in the presence of a metal fluoride catalyst. U.S. Pat. No. 5,672,786 discloses a vapor phase reaction for the production of HFC-32 by contacting HCC-30 with HF in the presence of a fluorination catalyst selected from the group consisting of the oxide, fluoride or oxyfluoride of at least one of chromium, aluminum, zinc, nickel, cobalt, copper and magnesium to produce a product stream comprising HFC-32, HCFC-31 and unreacted starting materials.
In U.S. Pat. No. 5,208,395 there is disclosed a vapor phase reaction for the production of HFC-32 by contacting HCC-30 with HF in the presence of certain relatively weak Lewis acid catalysts, such as tin (IV) and bismuth (III) salts, preferably chlorides, and especially tin tetrafluoride, on activated carbon.
There are various drawbacks with these processes. All of these processes require relatively high temperatures of between 200° C. to about 600° C. to make appreciable amounts of product. In practice, these processes for HFC-32 production suffer from a variety of problems including low product yield and poor product selectivity, as well as operational difficulties such as feed decomposition and the reaction mixture and environment can be highly corrosive.
There is a need for a process for the production of difluoromethane (HFC-32), 1,1,1-trifluoroethane (HFC-143a) and 1,1-difluoroethane (HFC-152a) that are more economical and less corrosive than existing manufacturing methods and provides for the production of desired product(s) in good yield and selectivity even at relatively low temperatures. The relatively lower reaction temperatures than that of the previous inventions would enable one to minimize the problem of feed decomposition and corrosion.